Fix macOS client/server failing to start from terminal

The macOS client and server were completely unusable when run from a
terminal. They blocked forever in `[NSApp run];` which was called by
Sys_InitProtocolHandler(). `applicationDidFinishLaunching` was never
called to exit the NSApp run loop.

Use SDL's SDL_DROPFILE event to receive URLs to handle on macOS instead.
This also handles URLs while the game is running (connect to new server)
instead of nothing happening when clicking a link while the game is
running.

code/sys/sys_local.h

@@ -66,6 +66,5 @@ int Sys_PID( void );
 qboolean Sys_PIDIsRunning( int pid );
 
 #ifdef PROTOCOL_HANDLER
-char *Sys_InitProtocolHandler( void );
 char *Sys_ParseProtocolUri( const char *uri );
 #endif